Exhaust gas purifying system

ABSTRACT

In an exhaust gas purifying system having a catalyst provided in an exhaust pipe for carrying out chemical reduction reaction so as to remove NOx contained in exhaust gas from an engine, an additive injection valve is provided in the exhaust pipe at an upstream side of the catalyst for injecting additive agent into the exhaust pipe, so that the chemical reduction reaction at the catalyst is facilitated. An injection port of the additive injection valve is formed into a slit-shape, so that a sheet-like spray is formed when the additive is injected from the additive injection valve.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No.2007-253630, which is filed on Sep. 28, 2007, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an exhaust gas purifying system, suchas a urea SCR system (selective catalytic reduction), for purifyingexhaust gas of an internal combustion engine by specific purifyingreaction of the exhaust gas. The present invention further relates to anadditive injection valve for injecting addition agent in liquid formwhich will be used for the specific purifying reaction of the exhaustgas, and relates to an additive injection device.

BACKGROUND OF THE INVENTION

Recently, a urea SCR system (selective catalytic reduction) is underdevelopment and partly in practical use, as disclosed in Japanese PatentPublication No. 2003-293739, which will be applied to an electric powerplant, various kinds of factories, and vehicles (in particular, vehicleshaving diesel engines), as an exhaust gas purifying system for purifyingNOx (nitrogen oxides) contained in exhaust gas at a high purifyingratio.

The urea SCR system is composed of a catalyst for facilitating purifyingreaction of exhaust gas, an exhaust pipe for supplying exhaust gas froman exhaust gas generating device (for example, an internal combustionengine) to the catalyst, and an additive injection valve provided in theexhaust pipe for injecting urea aqueous solution (ammonia) into theexhaust gas flowing through the exhaust pipe. The catalyst facilitatesreduction reaction (the purifying reaction of exhaust gas) for NOx basedon the urea aqueous solution. According to the above structure, the ureaaqueous solution is supplied to the catalyst (provided at a downstreamside) together with the exhaust gas by use of the exhaust gas flow. As aresult, the reduction reaction of NOx based on the urea aqueous solution(ammonia) is generated at the catalyst to purify the exhaust gas. Moreexactly, the urea aqueous solution injected from the additive injectionvalve is hydrolyzed by heat of the exhaust gas to generate ammonia(NH3), and the NOx contained in the exhaust gas is reduced by theammonia on the catalyst, so that the exhaust gas is purified.

An exhaust gas purifying system is also known in the art, for example,as disclosed in Japanese Patent Publication No. 2001-3737, according towhich a center axis for injection of an additive injection valve 100 isarranged in parallel to a center axis 10 a of an exhaust pipe 10, asshown in FIGS. 14A and 14B. In this injection valve, multiple injectionports are circularly (or concentrically) provided at a nozzle portion101 thereof.

It is desired in the above exhaust gas purifying system that ammoniagenerated by the hydrolysis of the urea aqueous solution is supplied towhole area of the catalyst, so that NOx is reduced at the whole area ofthe catalyst to increase the purifying ratio of the catalyst.

According to the exhaust gas purifying system shown in FIGS. 14A and14B, a spray form of the urea aqueous solution injected by the additiveinjection valve 100 becomes a mid-air conical shape, because themultiple injection ports are circularly arranged. As a result, thesprayed ammonia (the urea aqueous solution) reaches at an annular area102 of an upstream side of SCR catalyst 20 (an annular hatched area inFIGS. 14A and 14B), whereas the sprayed ammonia (the urea aqueoussolution) may not reach at any other areas than the annular area 102 Insuch a case, reduction reaction for NOx differs from area to area on theSCR catalyst 20, and thereby the purifying ratio will be decreased.

SUMMARY OF THE INVENTION

The present invention is made in view of the above problems. It is anobject of the present invention to provide an exhaust gas purifyingsystem, an additive injection valve, and an additive injection device,according to which addition agent will be supplied to a larger area of acatalyst for facilitating purifying reaction of exhaust gas.

According to one of features of the present invention, an exhaust gaspurifying system for a vehicle has a catalyst provided in an exhaustpipe of an internal combustion engine for facilitating chemicalreduction reaction to purify NOx contained in exhaust gas from theengine. An additive injection valve is provided in the exhaust pipe atan upstream side of the catalyst for injecting additive agent into theexhaust pipe, so that the chemical reduction reaction at the catalyst isfacilitated. And an injection port of the additive injection valve isformed into a slit-shape, so that a sheet-like spray is formed when theadditive is injected from the additive injection valve.

According to another feature of the invention, the slit-shape injectionport is formed as a straightly extending slit.

According to a further feature of the invention, a width of theslit-shape injection port in a short side direction is not uniform.

According to a still further feature of the invention, the width of theslit-shape injection port at a position away from a center in alongitudinal direction is made smaller than the width of the slit-shapeinjection port at the center.

According to a still further feature of the invention, a center line ofthe injection by the additive injection valve is arranged to be inparallel to a center line of the exhaust pipe.

According to a still further feature of the invention, the slit-shapeinjection port crosses a center of the exhaust pipe and straightlyextends in a radial direction of the exhaust pipe.

According to a still further feature of the invention, DPF is providedin the exhaust pipe at an upstream side of the additive injection valve.

According to a still further feature of the invention, the exhaust pipehas a straight pipe portion, the catalyst is provided at a downstreamside of the straight pipe portion, and the additive injection valve isprovided at an upstream side of the straight pipe portion. And theadditive injection valve is provided in the straight pipe portion insuch a manner that a center line of a nozzle portion of the additiveinjection valve comes into line with a center line of the straight pipeportion.

According to a still further feature of the invention, a center line ofthe catalyst comes into line with the center line of the straight pipeportion.

According to a still further feature of the invention, the exhaust pipehas a first curved pipe at the upstream end of the straight pipeportion, and the additive injection valve is provided at the firstcurved pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a schematic view showing an SCR system according to anembodiment of the present invention;

FIG. 2 is a cross sectional view showing an additive injection valveaccording to the present invention;

FIG. 3 is a top plan view showing a plate member of the presentinvention;

FIG. 4 is a schematic perspective illustration for a spray form of ureaaqueous solution injected from the additive injection valve according tothe present invention;

FIG. 5 is a schematic view showing condition in which the urea aqueoussolution is supplied to a catalyst;

FIG. 6 is a top plan view showing a plate member of a comparisonexample;

FIG. 7 is a top plan view showing a plate member according to amodification of the present invention;

FIG. 8 is also a top plan view showing a plate member according toanother modification of the present invention;

FIG. 9 is a view showing velocity distribution of exhaust gas;

FIGS. 10 to 13 are schematic views respectively showing furthermodifications of the present invention; and

FIGS. 14A and 14B are schematic views showing a conventional urea SCRsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be hereinafter explainedwith reference to the drawings. In the embodiment of the presentinvention, the invention is applied to a diesel engine (exhaust gasgenerating device) mounted on a vehicle as an SCR system (an exhaust gaspurifying system).

A general outline of the SCR system will be explained with reference toFIG. 1. In FIG. 1, the SCR system purifies exhaust gas emitted from adiesel engine of a vehicle. A diesel particulate filter (DPF) 40, anexhaust pipe 10 and an SCR catalyst 20 are arranged in this order froman upstream of exhaust gas flow, so that the exhaust gas flows throughthe exhaust pipe 10, being swirled as indicated by an arrow A.

The DPF 40 is a filter of a continuous re-generative type forcontinuously trapping and removing diesel particulates (PM=ParticulateMatter) contained in the exhaust gas. For example, the trapped PM iscombusted (as a re-generation treatment) by a post fuel injection, whichis carried out after a main fuel injection. As a result, the DPF 40 iscontinuously used as the filter. The DPF 40 carries an oxidationcatalyst made of platinum or platinated metal for removing HC and COtogether with soluble organic constituent (SOF), which is one of PMcompounds.

An additive injection valve 30 is provided at the exhaust pipe 10, forinjecting (supplying) urea aqueous solution (addition agent) to exhaustgas flowing through the exhaust pipe 10. The urea aqueous solutioninjected from the additive injection valve 30 is converted to ammonia(NH3) by heat of the exhaust gas, and such converted ammonia is suppliedto the SCR catalyst 20 on a downstream side together with the exhaustgas so that they are used for exhaust gas purifying reaction.

The SCR catalyst 20 facilitates well-known reduction reaction (thepurifying reaction of the exhaust gas), and carries out chemicalreactions as indicated by the following reaction formulas (Formula 1 toFormula 3), so as to reduce NOx contained in the exhaust gas:

4NO+4NH₃+O₂→4N₂+6H₂O   (Formula 1)

6NO₂+8NH₃→7N₂+12H₂O   (Formula 2)

NO+NO₂+2NH₃+O₂→2N₂+3H₂O   (Formula 3)

The urea aqueous solution is drawn up by a pump (not shown) from a ureaaqueous solution tank (not shown), and supplied to the additiveinjection valve 30 through a supply pipe.

As shown in FIG. 1, the exhaust pipe 10 of an upstream side of the SCRcatalyst 20 is composed of a cylindrical straight pipe 11 connected tothe SCR catalyst 20, a first curved pipe 12 connected to the straightpipe 11 and bent into an arc shape, and a second curved pipe 13connected to the DPF 40 and bent into an arc shape, but bent in adirection opposite to that of the first curved pipe 12. A projectedportion 14 is provided at the first curved pipe 12 and the additiveinjection valve 30 is provided in the projected portion 14.

The projected portion 14 is formed into a cylindrical shape, and isopened to the first curved pipe 12 in such a manner that a center lineof the projected portion 14 comes into line with a center line 11 a ofthe straight pipe 11. In other words, the projected portion 14 is fixedto a curved outer peripheral portion of the first curved pipe 12 in sucha manner that the projected portion 14 is projected in a directionopposite to the straight pipe 11. The center line 11 a of the straightpipe 11 is also in line with a center line of the SCR catalyst 20. Theadditive injection valve 30 is attached to an end portion 14 a of theprojected portion 14 (at an opposite side of the straight pipe 11), sothat an injection port of the additive injection valve 30 is opened tothe inside of the projected portion 14. A center line of a nozzleportion of the additive injection valve 30 is also arranged to come intoline with the center line 11 a of the straight pipe 11. As a result ofthe above structure, injection direction of the urea aqueous solution bythe additive injection valve 30 is made to be in parallel to the centerline 11 a of the straight pipe 11.

A structure of the additive injection valve 30 will be explained withreference to FIG. 2. The additive injection valve 30 is anelectromagnetic valve basically having the same structure to a fuelinjection valve for a gasoline engine. A nozzle portion 31 formed at aforward end of the additive injection valve 30 has a needle 33, which isaccommodated in a valve body 32 in a sliding manner in an axialdirection. The needle 33 is seated on (or separated from) a valve seat34 formed in the valve body 32. An injection port 35 a is formed in aplate member 35, which is provided at a downstream side of the valveseat 34.

An electromagnetic solenoid 36 is arranged at an upper side of thenozzle portion 31. A terminal 37 is connected to the electromagneticsolenoid 36. An inlet port 39, communicated with a passage 38, which isformed between the valve body 32 and the needle 33, is connected to atank for storing the urea aqueous solution. According to such structure,the urea aqueous solution will be supplied to a valve seating portion ofthe valve seat 34 through the inlet port 39 and the passage 38.

When electric power is supplied to the above electromagnetic solenoid 36via the terminal 37 under control of ECU 41 (an electronic control unitmounted on the vehicle, see FIG. 1), the needle 33 is moved in a valveopening direction. As a result, the urea aqueous solution reaching atthe valve seating portion flows through an opened space between theneedle 33 and the valve seat 34 in a downstream direction. And the ureaaqueous solution having passed between the needle 33 and the valve seat34 is injected through the injection port formed in the plate member 35.

An effect of the SCR system of the present invention will be explainedwith reference to FIGS. 3 to 5. In the following explanation, it isassumed that the exhaust gas flows in the exhaust pipe 10 in a swirledform.

As shown in FIG. 3, a slit-shape injection port 35 a is formed in thedisc-shaped plate member 35. The slit-shape injection port 35 a isstraightly extending in a radial direction of the disc-shaped platemember 35 (the radial direction of the exhaust pipe 10), wherein thestraightly extending line for the injection port 35 a crosses a centerof the plate member 35 (a center of the nozzle portion 31). As shown inFIG. 4, the urea aqueous solution injected from the injection port 35 aforms a sheet-like spray. More exactly, the spray of the injected ureaaqueous solution flows towards the SCR catalyst 20, wherein, at abeginning of the injection, the injected spray is not rotated by theswirled flow of the exhaust gas due to penetration of injection.However, as the injected spray flows in the downstream directionthereafter, the injected spray starts with rotation by the swirled flowof the exhaust gas. As above, the injected spray of the urea aqueoussolution has a plate form at an area adjacent to the injection port 35a, but the shape of the injected spray is formed into a spiral shape ata position, which is separated by more than a predetermined distancefrom the injection port 35 a toward the SCR catalyst 20, wherein thespiral shape is formed depending on flow speed of the swirled flow ofthe exhaust gas. The spray of ammonia, which is generated by hydrolysisof the urea aqueous solution, (including the spray of the urea aqueoussolution) arrives at an upstream side surface 20 a of the SCR catalyst20, as indicated by a hatched portion in FIG. 5.

An area 21 of the upstream side surface 20 a of the SCR catalyst 20, atwhich the spray of ammonia (the urea aqueous solution) arrives, has ashape corresponding to the slit-like shape of the injection port 35 a.Namely, the area 21 (also referred to as the additive agent arrivingarea) is formed as a rectangular area extending in the radial directionof the exhaust pipe 10. This is because the center line of the injectionby the additive injection valve 30 is arranged to be in parallel to thecenter line of the exhaust pipe 10. In other words, the center lines ofthe nozzle portion 31, the SCR catalyst 20 and the straight pipe 11 areso made as to come into line with each other, and furthermore theinjection direction of the urea aqueous solution by the additiveinjection valve 30 is arranged to be in parallel to the center line 11 aof the straight pipe 11. According to such arrangement, the crosssectional shape of the spiral spray of the urea aqueous solution (theshape in the cross section extending in the radial direction of theexhaust pipe 10) is expanded, as a distance from the injection port 35 abecomes larger as a result of spreading the injected spray. However, thecross sectional shape still corresponds to the slit-like shape of theinjection port 35 a. As above, the additive agent arriving area 21 hasthe shape corresponding to the slit-like shape of the injection port 35a.

The area 21 (the additive agent arriving area 21) of the upstream sidesurface 20 a of the SCR catalyst 20, at which the spray of ammonia (theurea aqueous solution) arrives, is changed from time to time as the flowspeed (flow speed in an axial flow direction and flow speed in arotational direction) of the swirl flow of the exhaust gas varies. Forexample, the additive agent arriving area 21 is rotated in acircumferential direction around the center line of the exhaust pipe 10,as indicated by dotted lines in FIG. 5.

The above embodiment has the following advantages.

According to the above embodiment, the injection port 35 a of the nozzleportion 31 is formed into the slit-shape, the center lines of the nozzleportion 31 and the SCR catalyst 20 and the center line 11 a of thestraight pipe 11 are so made as to come into line with each other, andthe injection direction of the urea aqueous solution by the additiveinjection valve 30 is arranged to be in parallel to the center line 11 aof the straight pipe 11. As a result, the area 21 (the additive agentarriving area 21) of the upstream side surface 20 a of the SCR catalyst20, at which the spray of ammonia (the urea aqueous solution) arrives,is formed as the rectangular area continuously extending in the radialdirection of the exhaust pipe 10, as indicated by the hatched area inFIG. 5. In addition, the additive agent arriving area 21 is changed fromtime to time as the flow speed of the swirl flow of the exhaust gasvaries. Therefore, as explained above, the additive agent arriving area21 is rotated in a circumferential direction around the center line ofthe exhaust pipe 10, as indicated by the dotted lines in FIG. 5. As aresult, ammonia can be supplied to a wide area of the SCR catalyst 20.

Furthermore, since the injection port 35 a is formed into the slit-shapeextending in the radial direction of the exhaust pipe 10, density of thespray for ammonia (the urea aqueous solution) arriving at the additiveagent arriving area 21 can be uniformized. More exactly, the density ofthe spray for ammonia becomes more uniform, when compared with acomparison example shown in FIG. 6, in which multiple small injectionports 103 are provided in the plate member 35.

(Modifications)

The present invention is not limited to the above explained embodiment,but may be modified in the following manners.

(a) In the above embodiment, the injection port 35 a is formed into theslit-shape, which straightly extends in the radial direction of theplate member 35 passing through the center thereof. The slit-shapeinjection port 35 a may be, however, offset from the center of the platemember 35. The slit-shape injection port 35 a may be formed into abent-shape. Furthermore, the injection port 35 a may be formed into across-slit-shape, which extends in the radial direction from a certainpoint (e.g. the center) of the plate member, as shown in FIG. 7.

(b) In the above embodiment, the slit-shape injection port 35 a has arectangular shape, so that a width in a short-side direction (a slitwidth) has a predetermined constant value. The slit width of theinjection port 35 a may be, however, changed along a longitudinaldirection. In case of the slit-shape injection port, spray condition(for example, particle diameter, penetration, etc.) of the urea aqueoussolution injected from the injection port 35 a varies at positions inthe longitudinal direction of the slit. Accordingly, the spray conditionmay be adjusted by setting the width in the short-side direction (theslit-width) of the injection port 35 a for respective longitudinalpositions.

For example, in the case that particulate diameter of the urea aqueoussolution tends to become larger at a longitudinal position, a distanceof which is longer away from the center of the plate member, theparticulate diameter of the spray for the urea aqueous solution can bemade uniform by adjusting the slit-width in such a manner that theslit-width is made smaller as the longitudinal position is more awayfrom the center, as shown in FIG. 8.

The velocity distribution of the exhaust gas (a result of measurement,in which flow speed of the exhaust gas in the axial direction of theexhaust pipe 10 is measured at respective points in the radial directionand at the same time) shows a certain tendency, as shown in FIG. 9, thatthe flow speed varies depending on a distance in the radial directionfrom the center line 10 a of the exhaust pipe 10. The flow speed becomesat its maximum at an intermediate point between the center line 10 a ofthe exhaust pipe 10 and an inner surface 10 b. The flow speed isdecreased at a point coming closer to the inner surface 10 b of theexhaust pipe 10. In FIG. 9, arrows designate vectors for the flow speedof the exhaust gas flowing in the exhaust pipe 10. Accordingly, thepenetration of the spray for the urea aqueous solution may be adjustedby setting the width in the short-side direction (the slit-width) of theinjection port 35 a in accordance with the above velocity distribution.

(c) In the above embodiment, the additive injection valve 30 is providedat the projected portion 14 of the first curved pipe 12. However, asshown in FIG. 10, the additive injection valve 30 may be provided at acorner of an L-shaped pipe.

(d) In the above embodiment, the urea aqueous solution is injected inparallel to the center line of the exhaust pipe 10. More exactly, theadditive injection valve 30 is provided in such a manner that theinjection direction (the center line of injection) of the urea aqueoussolution by the additive injection valve 30 becomes in parallel to thecenter line 11 a of the straight pipe 11. The present invention is notlimited to such embodiment. For example, as shown in FIGS. 11 and 12,the center line of the injection by the additive injection valve 30 maybe inclined by a predetermined angle with respect to the center line 10a of the exhaust pipe 10.

(e) In the above embodiment, the additive injection valve 30 isdisclosed as an example of a spray forming means, according to which theadditive is supplied to the passage 38 (the passage from the inlet port39 to the injection port 35 a, as shown in FIG. 2) and the additive isinjected by opening and closing the passage 38. The spray forming meansmay be arranged such that gas, in which the additive is compressed (suchas, compressed air), maybe injected. For example, as shown in FIG. 13,the exhaust gas purifying system may have an additive supply device 52,a compressed air supply device 53, a mixing device 54, and an injectionnozzle 55, wherein the additive and the compressed air respectivelysupplied from the additive supply device 52 and the compressed airsupply device 53 are mixed in the mixing device 54, and the mixed gas isinjected from the injection nozzle 55. An injection port of theinjection nozzle 55 is made as a slit-shape port, so that a sheet-likespray of the additive is formed.

(f) In the above embodiment, the present invention is applied to theurea SCR system for the engine mounted on the vehicle. The presentinvention is not limited to such application, but may be applied to anyother system for purifying the exhaust gas by use of the additive andthe catalyst.

1. An exhaust gas purifying system for a vehicle comprising: a catalystprovided in an exhaust pipe of an internal combustion engine forfacilitating chemical reduction reaction to purifying NOx contained inexhaust gas from the engine; and an additive injection valve provided inthe exhaust pipe at an upstream side of the catalyst for injectingadditive agent into the exhaust pipe, so that the chemical reductionreaction at the catalyst is facilitated, wherein an injection port ofthe additive injection valve is formed into a slit-shape.
 2. The exhaustgas purifying system according to the claim 1, wherein the slit-shapeinjection port is formed as a straightly extending slit.
 3. The exhaustgas purifying system according to the claim 1, wherein a width of theslit-shape injection port in a short side direction is not uniform. 4.The exhaust gas purifying system according to the claim 3, wherein thewidth of the slit-shape injection port at a position away from a centerin a longitudinal direction is made smaller than the width of theslit-shape injection port at the center.
 5. The exhaust gas purifyingsystem according to the claim 1, wherein a center line of the injectionby the additive injection valve is arranged to be in parallel to acenter line of the exhaust pipe. 6 The exhaust gas purifying systemaccording to the claim 1, wherein the slit-shape injection port crossesa center of the exhaust pipe and straightly extends in a radialdirection of the exhaust pipe.
 7. An exhaust gas purifying system for avehicle comprising: a catalyst provided in an exhaust pipe of aninternal combustion engine for facilitating chemical reduction reactionto purify NOx contained in exhaust gas from the engine; and a sprayforming means provided in the exhaust pipe at an upstream side of thecatalyst for injecting additive agent into the exhaust pipe, so that thechemical reduction reaction at the catalyst is facilitated, wherein asheet-like spray is formed when the additive is injected from the sprayforming means.
 8. The exhaust gas purifying system according to theclaim 1, further comprising: DPF provided in the exhaust pipe at anupstream side of the additive injection valve or the spray formingmeans.
 9. The exhaust gas purifying system according to the claim 1,wherein the exhaust pipe has a straight pipe portion, the catalyst isprovided at a downstream side of the straight pipe portion, the additiveinjection valve is provided at an upstream side of the straight pipeportion, and the additive injection valve is provided in the straightpipe portion in such a manner that a center line of a nozzle portion ofthe additive injection valve comes into line with a center line of thestraight pipe portion.
 10. The exhaust gas purifying system according tothe claim 9, wherein a center line of the catalyst comes into line withthe center line of the straight pipe portion.
 11. The exhaust gaspurifying system according to the claim 9, wherein the exhaust pipe hasa first curved pipe at the upstream end of the straight pipe portion,and the additive injection valve is provided at the first curved pipe.